23. Visibility graphs

a. Definitions; application to shortest paths
b. Rotational sweep at each vertex: O(n^2 log n) time
c. Dual interpretation as sweep over line arrangement
d. Topological sweep: O(n^2) time
e. Maintaining trapezoidal decomposition: O((n+k) log n) time
f. Flips in pseudo-triangulations (pictures only): O(n log n+ k) time.